This invention relates to telephony and more particularly to a receiver for demultiplexing pulse code modulated (PCM) information transmitted over a telephone line.
In general PCM receivers are connected to one end of a telephone transmission line which carries time multiplexed, pulse code modulated information in a continuous serial stream of bipolar pulses. The stream of data pulses or bit stream is generated by sequentially sampling and digitizing a number of analog data channels, usually 24 channels, at the transmitting end of the telephone line. The bit stream is divided into frames consisting of 24 channels, each having an 8-bit channel word for a total of a 192 bits per frame. A 193rd bit or frame bit (also called an "S" bit) is transmitted at the end of each frame and is used to assure that the receiver's demultiplexing circuitry is properly synchronized or framed with respect to the bit stream.
In order to synchronize or frame the receiver, the frame bit (the 193rd bit of each frame) is transmitted in a pre-determined, reoccurring code or pattern such as 11001100 and so one. Conventionally, because of the redundancy of such a reoccurring frame bit pattern, only every other frame bit (referred to as the terminal frame bit) is used to synchronize the receiver, and the alternate frame bit (referred to as the signaling frame bit) is used to identify the one frame in six for which each 8-bit channel word has seven bits of digitized data information and one bit of signaling information, such as dialing information.
In addition to identifying the one frame in six which contains signaling information, some of the signaling frame bits can themelves be used as data bits for transmitting low speed information, such as alarm information.
Having appropriated the signaling frame bit for identifying the signaling frame and in some cases for use as a data bit, prior art PCM receivers are left with only the terminal frame bit (alternate framing bits having a reoccurring 1010 pattern) for establishing framing of the receiver. Because framing in conventional PCM receivers is dependent on the reoccurring 1010 pattern of the terminal frame bit, there exists a likelihood that such receivers will erroneously frame on certain common interference signals which have been induced into the transmission system.
For example, a 2 khz interference signal, which results from the presence of power transmission lines in the immediate vicinity of the data transmission system, can in conventional PCM receivers, induce erroneous or false framing. Because the sequential data sampling rate of the 24 channels at the data transmission end is 8 khz, the sign bit of each 8-bit channel word which results from sampling a 2 khz interference tone varies in a continous pattern of 11001100 and so on. Because the continuous sign bit pattern of the 2 khz interference tone may exist in any one of the 24 channel words throughout the length of a frame, there exists the possibility that the framing circuitry in the receiver might erroneously lock onto the 2 khz tone's sign bit instead of the correct terminal frame bit.
The prior art does not provide a solution to the problem of false framing in the presence of a 2 khz interference tone. For example, U.S. Pat. No. 3,742,139 issued to Boehly et al. discloses using only the terminal frame bit to establish framing. The Boehly system simply compares the terminal frame bit pattern through eight consecutive frames searching for the expected 1010 combination. If the expected bit pattern is present through eight consecutive frames, according to the Boehly patent, there is "little or no chance of false synchronization". The Boehly device, however, could erroneously lock onto a 2 khz interference tone. Such an occurrence would completely disrupt that system's ability to carry data.
U.S. Pat. No. 3,909,540 issued to Maryscuk, et al. provides a two level reframing procedure in which the reframing circuit first checks for the 1010 pattern of the terminal frame bit and then before framing is established, checks for a combination of three consecutive binary 1's for the frame bits in frames 8, 9 and 10. The Maryscuk device, therefore, gives some cross-checking between the terminal frame bit and the signaling frame bit to determine that the framing is proper. The Maryscuk, et al. circuitry might, however, erroneously lock on a 2 khz tone if the data words in frames 8, 9 and 10 after the sign bit for the 2 khz tone happened to have the expected combination of three consecutive "1". Such an occurrence is not altogether unlikely. In any event the framing time required by the Maryscuk, et al. device is significantly increased by the need to locate the three binary 1's in the frame bit pattern before reframing is completed.
It is, therefore, an object of the present invention to overcome the problem of false framing in the presence of a 2 khz interference tone by providing a framing circuit having a false framing detector so that the receiver does not erroneously frame on the sign bit of the 2 khz tone.
It is also an object of the present invention to provide two additional low speed data channels carried via the signaling frame bit while at the same time providing assurance against false framing.